If you send the tractor in for service or repair they pressure wash it before they send it back. Just don't do it when the engine is hot. If you spray cold water on a hot injection pump you can shrink the housing and the hot guts can bind up and destroy themselves. You don't want to spray cold water on a hot exhaust, either. You can crack a manifold. Water can get into electrical connections and cause problems but that can happen on anything. It would be a good idea to figure out where the transmission and axle vents are located and avoid spraying water directly on them, or, more precisely, up under them. Just use a little common sense. If you wash the radiator out with a pressure washer, do it from a distance so you don't flatten out the fins. I'm sure others will come up with things to watch out for. Remember, the guy working the washrack at the dealership ain't a master tech, they hire some kid, point out the rack and tell him to wash. If there was that many things to go wrong it would cost them so it must be pretty safe to let 'er rip!